Aug 24, 2014 - The star marks the hy- pocenter. Red dots ... of the fault marks the surface rupture from field mapping. (b) Slip ..... Roy Tam, and Jack Tomey.
The M w 6.0 24 August 2014 South Napa Earthquake by Thomas M. Brocher, Annemarie S. Baltay, Jeanne L. Hardebeck, Fred F. Pollitz, Jessica R. Murray, Andrea L. Llenos, David P. Schwartz, James Luke Blair, Daniel J. Ponti, James J. Lienkaemper, Victoria E. Langenheim, Timothy E. Dawson, Kenneth W. Hudnut, David R. Shelly, Douglas S. Dreger, John Boatwright, Brad T. Aagaard, David J. Wald, Richard M. Allen, William D. Barnhart, Keith L. Knudsen, Benjamin A. Brooks, and Katherine M. Scharer INTRODUCTION The M w 6.0 South Napa earthquake, which occurred at 10:20 UTC 24 August 2014 was the largest earthquake to strike the greater San Francisco Bay area since the M w 6.9 1989 Loma Prieta earthquake. The rupture from this right-lateral earthquake propagated mostly unilaterally to the north and updip, directing the strongest shaking toward the city of Napa, where peak ground accelerations (PGAs) between 45%g and 61%g were recorded and modified Mercalli intensities (MMIs) of VII–VIII were reported. Tectonic surface rupture with dextral slip of up to 46 cm was observed on a 12.5 km long segment, some of which was along a previously mapped strand of the West Napa fault system, although the rupture extended to the north of the mapped Quaternary strand. Modeling of seismic and geodetic data suggests an average coseismic slip of 50 cm, with a maximum slip of about 1 m at depths of 10–11 km. We observed up to 35 cm of afterslip along the surface trace in the week following the mainshock, primarily along the southern half of the surface rupture that experienced relatively little coseismic offset. Relocation of the sparse aftershock sequence suggests en echelon southwest- and northeastdipping fault planes, reflective of the complex fault geometry in this region. The Napa basin and historic and late Holocene alluvial flood deposits in downtown Napa amplified the ground motions there. Few ground failures were mapped, reflecting the dry season (as well as a persistent drought that had lowered the groundwater table) and the short duration of strong shaking in the epicentral area.
TECTONIC SETTING OF THE SOUTH NAPA EARTHQUAKE The South Napa fault rupture lies within an 80 km wide set of major north-northwest-trending faults of the San Andreas fault system, forming the boundary between the Pacific and doi: 10.1785/0220150004
North American tectonic plates (Jennings, 1994; Fig. 1). The West Napa fault system is a relatively minor strike-slip fault in the Contra Costa shear zone, which transfers slip between the Northern Calaveras, West Napa, and Concord faults (Unruh and Kelson, 2002a,b; Kelson et al., 2004, 2005; Wesling and Hanson, 2008; Brossy et al., 2010). Block modeling of Global Positioning System (GPS) data estimated a slip rate of 4:0 ! 3:0 mm=yr on the West Napa fault (d’Alessio et al., 2005). In the Uniform California Earthquake Rupture Forecast (UCERF 3) model, the entire Contra Costa shear zone was assigned a maximum slip rate of 1 mm=yr (Field et al., 2013, appendix C, 18 pp). The earthquake was located near the eastern shore of San Pablo Bay, midway between two major active fault systems: the Hayward–Rodgers Creek fault system 12 km to the west and the Concord–Green Valley fault system 13 km to the east. The earthquake epicenter lies 1.7 km west of the main mapped surface trace of the West Napa fault system and close to the surface traces of the lesser known Carneros and Franklin faults (Graymer et al., 2002). Although several faults are mapped in the vicinity of the earthquake, only the West Napa fault system is known to have displaced Holocene-age sediments (Wesling and Hanson, 2008). The West Napa fault system forms the western margin of a basin that underlies much of Napa Valley. The basin, filled with Cenozoic sedimentary and volcanic deposits, is ∼2 km deep beneath the city of Napa. The South Napa earthquake occurred where the prominent east-facing gravity and magnetic gradients associated with the main bedrock strand of the West Napa fault system diminish and are replaced by prominent west-facing gravity gradients that mark the eastern margin of the San Pablo Bay basin and the Carneros and Franklin faults (Langenheim et al., 2006). The 2000 M w 4.9 Yountville earthquake was attributed to the main bedrock strand of the West Napa fault system 20 km north-northwest of the South Napa epicenter (Langenheim et al., 2006). Modeling of potential-field data, coupled with aftershock locations, indicates a
Seismological Research Letters
Volume 86, Number 2A
March/April 2015
309
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